The authorities tightened security at key installations across the country following the powerful blast at the airport in Davao city on Tuesday which also injured more than 150 people.

Abu Sayaf warns of more attacks

A leader of the Abu Sayyaf told the local ABS-CBN television on Wednesday that his group staged the bombing although the group said it regretted there were many casualties. It warned of more attacks to cripple the economy.

The Abu Sayyaf, noted for its kidnappings and beheading of victims who fail to pay ransom, had been classified a terrorist group by Washington for its alleged links to Osama bin Laden's al-Qaeda network.

A regional terror group, the Jemaah Islamiyah (JI), which foreign governments say is the Southeast Asian chapter of the al-Qaeda, has also been accused of carrying out bombings in the region, including the Bali attack in October last year.

Govt "not taking claim seriously"

The Philippine government said it was not taking the Abu Sayyaf claim for the blast seriously, saying that Davao, the largest city and commercial capital of the southern Philippines, was not part of the group's terrain.

"We should not take that claim very seriously. They operate in another area," Defence Secretary Angelo Reyes said after briefing President Gloria Arroyo, who flew in early on Wednesday to oversee the investigations.

Moro Islamic Liberation Front members held for attack

Interior Secretary Jose Lina said separately that five members of the Moro Islamic Liberation Front (MILF), the country's largest Muslim separatist group, have been held in connection with the bombing.

It was the worst blast in Southeast Asia since the bomb attack on the Indonesian island of Bali in October last year killed 202 people.

The Davao attack came amid a backdrop of increased Muslim guerrilla activity in the region ahead of a planned deployment of US anti-terror troops.

MILF denies involvement

The MILF on Tuesday denied responsibility for the blast and said it was even prepared to be involved in any probe to identify the perpetrators of the crime.

"They (five suspects) are identified as members of the MILF," Lina told reporters. "They are in the order of battle of the police and military."

Lina said the five MILF members were part of a "network of bombers," apparently referring to a series of suspected MILF bomb blasts in the south after the military overran a MILF enclave last month.

The military offensive left almost 200 people dead, most of them MILF fighters. The suspected MILF bomb blasts after the military blitz toppled several electricity transmission towers and plunged much of the southern Philippines into darkness in recent weeks.

The 12 500-strong MILF has been waging a 25-year campaign to set up an Islamic state in the south of mainly Roman Catholic Philippines.

Investigators have "some success

Reyes said investigators have achieved "some success" in tracking down the perpetrators of the airport bombing.

An hour after the blast on Tuesday, another bomb went off near a government health centre in Tagum town just outside Davao, injuring three more people.

Security officials in Manila said they were boosting security around oil depots, seaports and other vital installations in the capital and other major cities.